Application 2026
Hydrogen positions in magnesium borohydride ammoniates
Torben René Jensen
Aarhus University, Aarhus
Solid state hydrogen materials have been investigated intensively during the last decade as a
result of their high hydrogen content [1]. This is attractive, because hydrogen is a promising
energy carrier in a future society based on renewable energy. However, hydrogen is typically
bonded too strongly by covalent/metallic/ionic bonds in the major part of the known solid
state hydrogen storage materials. This gives rise to high operating temperature for hydrogen
release and uptake.
Recently, dihydrogen bonds between partly negative hydrogen from for example BH4- and
partly positive hydrogen atoms from for example NH3 have shown promising properties.
Unlike the classical hydrogen bonds, the dihydrogen bonds can react in the solid state via
elimination of hydrogen by exchanging the weak Hδ+•?••?••?••??δH interactions for strong
covalent bonds, and thus may open new routes to rational design of structures and hydrogen
release reaction mechanisms. Among the most classical systems containing dihydrogen bonds
are the amide-hydride system, LiNH2-LiH [2] and ammonia borane, NH3BH3 [3].
Metal borohydride ammoniates, M(BH4)m•??nNH3, is a newer class, which has been studied
for hydrogen storage purposes since 2009. From powder X-ray diffraction, Soloveichik et al.
solved the crystal structures of two new magnesium borohydride ammoniates, namely
Mg(BH4)2•??6NH3 and Mg(BH4)2•??2NH3 [4]. In Mg(BH4)2•??6NH3, Mg2+ is
octahedrally coordinated by NH3, while the BH4- groups act as counter ions. On the other
hand, Mg2+ is tetrahedrally coordinated by two NH3 and two BH4- groups in
Mg(BH4)2•??2NH3, and the structure is built of layers mediated by dihydrogen bonds.
Whereas Mg(BH4)2•??6NH3 releases NH3 (not suitable for hydrogen storage applications),
mainly hydrogen is released from Mg(BH4)2•??2NH3 which may reflect the importance of
the dihydrogen bond.
A range of new metal borohydride ammoniates with desirable hydrogen storage properties
have been synthesized since 2009 [5,6]. However, the crystal structures are all solved from Xray diffraction, which does not sufficiently determine the hydrogen positions. Indeed, these
positions and the dihydrogen interaction are of high interest, since they play a crucial role
tuning the compounds given rise to hydrogen release rather than ammonia release. Therefore,
we would to investigate metal borohydride ammoniates by neutron diffraction for the first
time; i.e. to accurately determine the hydrogen positions for Mg(11BD4)2•??2ND3 and
Mg(11BD4)2•??6ND3 in order to better understand the dihydrogen interaction. The PUS
instrument at the JEEP II infrastructure should be well-suited for this task. Samples with
deuterium and 11-B are prepared due to the high cross sections for incoherent neutron
scattering and neutron absorption for natural hydrogen and natural boron, respectively.
References
1. S. I. Orimo, Y. Nakamori, J. R. Eliseo, A. Züttel, and C. M. Jensen, Chem. Rev., 2007,
107, 4111•??4132.
2. P. Chen, Z. Xiong, J. Luo, J. Lin, and K. L. Tan, Nature, 2002, 420, 302•??304.
3. W. T. Klooster, T. F. Koetzle, P. E. M. Siegbahn, T. B. Richardson, and R. H. Crabtree, J
Am Chem Soc, 1999, 121, 6337•??6343.
4. G. Soloveichik, J.-H. Her, P. W. Stephens, Y. Gao, J. Rijssenbeek, M. Andrus, and J.-C.
Zhao, Inorg Chem, 2008, 47, 4290•??4298.
5. Y. Guo, H. Wu, W. Zhou, and X. Yu, J. Am. Chem. Soc., 2011, 133, 4690•??4693.
6. Y. Guo, X. Yu, W. Sun, D. Sun, and W. Yang, Angew Chem Int Ed, 2011, 50,
1087•??1091.